1. Field of the Invention
This invention relates to the bending of sheets of thermoplastic material, especially glass. Many techniques for bending glass sheets are known. Perhaps the simplest technique is to heat a glass sheet while supported on a contoured mold and to permit the glass sheet to sag into conformity with the mold. However, it is often desirable to speed the process by applying a mechanical bending force such as by pressing the glass between a pair of contoured molds. Another desirable process for shaping glass sheets is the recently invented process of drop forming, to be discussed later.
2. Description of Patents of Interest
U.S. Pat. No. 3,223,501 to J. C. Fredley et al discloses a gas hearth bed that uses hot gas for supporting and heating glass sheets to their deformation or tempering temperature while the sheets are conveyed along a furnace. Bottom surface heating by convection of hot gas and radiation of heat from the hot gas bed is supplemented by radiant heaters that irradiate the upper surface in an attempt to equalize top and bottom surface heat as a means to avoid thermal warpage. Electrical heaters may be used as a source of radiant heat in this patent. While temperature control is a factor in this patent to develop an adequate temper in the glass sheets treated, no harm results to the optical properties of the glass sheet surface treated by this patented apparatus if the maximum glass sheet temperature should exceed the minimum temperature needed for tempering, because the major surfaces of the glass sheet are maintained out of contact with solid members in this patent.
U.S. Pat. No. 3,396,000 to F. J. Carson et al discloses a method of bending and tempering glass sheets in which the sheets are shaped by warping due to a differential application of coolant to the opposite major sheet surfaces of a glass sheet immediately after the sheet is heated to substantially the softening point of the glass.
A particularly advantageous press bending process is shown in U.S. Pat. No. 3,846,104 to S. L. Seymour wherein a horizontally oriented glass sheet is heated and lifted by a lower bending mold into contact with an upper bending mold where it is retained by vacuum while the lower bending mold retracts. Then a tempering ring receives the bent glass sheet and conveys it from the bending station into a tempering station. That arrangement is especially advantageous in that it provides contoured support for the bent glass sheet during the tempering step and frees the bending station for initiating the next bending cycle while tempering of the first glass sheet is being carried out. A drawback to such an arrangement is that the major components of the apparatus, the upper and lower forming molds and the tempering ring, must all be custom fabricated for each different shape produced on such a bending and tempering line. It would be desirable not only to reduce the cost of fabricating these elements with each shape change, but also to reduce the down time of the bending and tempering line necessitated by the installation of these elements with each product change.
In U.S. Pat. No. 3,713,799 to H. A. McMaster, a similar arrangement is disclosed, but in which the lower shaping mold serves to carry the bent glass sheet into the tempering station, thus delaying the commencement of the next bending cycle until the lower shaping mold deposits the glass sheet in the tempering station and returns to the bending station. Likewise, in this arrangement a product change requires a major retooling of the bending station since the upper and lower forming molds as well as the gas support block into which the lower forming mold recesses, all must conform to the shape of the glass sheets being processed. A similar arrangement in U.S. Pat. No. 3,573,899 to H. A. McMaster et al. has the same drawback.
U.S. Pat. Nos. 3,507,639 to S. L. Seymour and 3,676,098 to H. R. Hall both show horizontal press bending arrangements wherein only two elements, the upper and lower bending molds, need to be custom fabricated for each glass shape being produced. It would be desirable to reduce the number of custom made parts even further. Furthermore, in both of these arrangements the edges of the bent glass sheets are not supported as they are conveyed from the bending station into the tempering station.
U.S. Pat. No. 3,476,540 to Ritter et al. discloses a glass bending arrangement whereby the inertia of a single vertically rising lower bending mold effects the bending. Disadvantageously, the bent glass sheets produced by this patented apparatus must pass without edge support along a roller conveyor into the tempering zone.
U.S. Pat. No. 3,600,150 to Rougeux shows a glass bending arrangement wherein a heat-softened glass sheet is slipped from a roller conveyor onto a flexible hammock and thereafter press bent between upper and lower forming molds. The purpose of the flexible hammock is to support the glass sheet initially out of contact with the rigid shaping mold surfaces. It is apparent that a major reconstruction of the apparatus would be required when a change in the glass shape is desired.
U.S. Pat. No. 4,092,141 to Frank et al discloses glass sheet shaping apparatus comprising a vacuum mold having a downwardly facing, apertured wall of metal covered with a refractory material such as fiber glass. The fiber glass cover is interposed between the convexly curved, downwardly facing, apertured wall of the vacuum mold and the upper surface of the shaped glass sheet, which is concavely curved in elevation. The surface of a glass sheet that is curved concavely develops a surface stress that increases the resistance of the glass sheet to develop surface distortion.
U.S. Pat. No. 4,202,681 to McMaster et al covers a vacuum holder system and a method for its use in bending glass. It discloses a three step method of using the system by first applying a high vacuum to engage a glass sheet against an apertured bottom plate of a vacuum holder, then applying a reduced vacuum to reduce the likelihood that the hot glass sheet would deform against the openings in the bottom plate of the vacuum holder and finally, applying a positive pressure to help separate the glass sheet from the vacuum holder. There is no teaching in the patent of covering the apertured bottom plate with a plurality of layers of porous fiberglass cloth to protect the major surface of the glass sheet from deforming into the apertures of the bottom plate when held thereagainst by suction.
U.S. Pat. No. 4,204,854 to McMaster et al teaches applying upwardly blown gas from beneath the spaces between rollers of a roller conveyor to lift a glass sheet into engagement against the downwardly facing, apertured surface of a vacuum holder. In this patent, the glass sheet is transferred from the vacuum holder to a ring mold having a shape that corresponds to the periphery of the sheet of bent glass. The holder may include a flat ring (see FIG. 8) or a curved ring 48 (see FIG. 9). This patent uses the upward gas flow to lift the glass sheet off the roller conveyor and onto the vacuum holder. Vacuum may be applied supplementary to the upward lift provided by the upward gas flow. This patent also finds it necessary to control the supplemental suction to avoid developing upward deformations in the hot glass sheet in those regions that engage the perforations of the vacuum plate by suction. This patent also fails to teach covering the apertured plate with a plurality of layers of porous fiberglass cloth.
Whenever it becomes necessary to develop a vacuum cycle comprising a high vacuum step followed by a lesser vacuum step, a complication is introduced into the control system for the vacuum cycle. The change in vacuum must be timed precisely with the engagement of the hot glass sheet by the apertured plate of the vacuum holder. Since glass sheets arriving for treatment at the shaping station are not of uniform thickness from sheet to sheet, lighter, thinner sheets are engaged more rapidly than heavier, thicker glass sheets. Hence, an automatic timing system to control a vacuum cycle would engage thinner glass sheets more susceptible to surface distortion than thicker glass sheets for a longer period at a higher vacuum than is necessary to engage the thinner glass sheets. Therefore, the latter are more likely to develop surface distortion. Furthermore, if the timing cycle is adjusted to reduce the duration of the high vacuum to conform to the requirements of thinner glass sheets, such a modified cycle might cause the vacuum to be reduced before a thicker glass sheet is engaged sufficiently for the reduced vacuum to suffice to maintain the thicker hot glass sheet in engagement against the downwardly facing, apertured, lower plate of the vacuum holder. Consequently, under such circumstances, a heavier glass sheet may drop by gravity before a ring-like member arrives in a proper position beneath the vacuum-engaged glass sheet. Such premature dropping is likely to interrupt operations until the glass is removed and the shaping station cleared for further operations.
It would be beneficial for the glass sheet shaping art to develop method and apparatus that improves the optical properties of the shaped glass sheets and also reduces the duration and frequency of breakdowns in operations that result from improper timing of the steps in the vacuum applying portion of a glass sheet shaping cycle. This benefit increases in importance in the handling of thin glass sheets (1/8 inch or 3.2 mm thick or less) that are more readily distorted and whose ratio of weight variances to total weight is more than for thicker glass sheets.